Bark beetle outbreaks have left a legacy of tree mortality across fire-prone landscapes of western North America that could influence how these forests will burn and regenerate. While research has been focused on forests in the western United States, the potential interactions between mountain pine beetle (MPB) outbreaks and wildfires have yet to be investigated in the sub-boreal forests of British Columbia that have pervasive tree mortality.
In this dissertation, I explored the relationships between MPB outbreaks and wildfires through field-based approaches that evaluate the drivers of fire severity, legacy structures, and seedling recruitment, and through satellite-based approaches that evaluated the drivers of burn severity. The majority of this research focused on three fires that burned in 2012, 2013, and 2014 across two provincial parks in central interior British Columbia that are managed as wilderness, are dominated by lodgepole pine (Pinus contorta var. latifolia), and situated at the epicenter of the outbreaks in these sub-boreal forests. I examined the influence of outbreak severity on fire severity using typical first-order fire effects measured in the field as well as legacy structure related to the consumption of woody biomass on snags/trees. I evaluated post-fire seedling recruitment across a gradient of fire severity, including fire refugia - areas only affected by MPB outbreaks and absent of recent fire. I conducted a landscape-scale assessment of MPB outbreak and fire by developing satellite maps of outbreak and fire severity to evaluate the drivers of high burn severity across these landscapes. I supplemented this research with a chemical analysis of pyrogenic carbon on snags from the Pole Creek Fire that burned through lodgepole pine forest with outbreak mortality in the eastern portion of the Cascade Range in Oregon.
In my field study, MPB outbreak severity did not influence fire severity as it related to immediate first-order fire effects, with the exception of a high probability of formation of deep char. Related to this increased formation of deep char, outbreak severity substantially influenced fire severity by affecting postfire legacy structure in these forests. My findings indicate that fire weather and topography are largely influencing fire severity related to first-order fire effects, while biomass consumption related to the architecture of structural legacies that remain post-fire is strongly linked to the status of trees at time of fire (live trees versus deadwood). These results highlight important synergistic effects of outbreak severity and fire severity. Post-fire recruitment density of lodgepole pine was related to fire severity and the abundance of serotinous cones on nearby lodgepole trees, demonstrating that fire is a key mechanism for seed release. Comparison of burned sites to fire refugia with substantial MPB outbreak mortality showed that fire is critical for regeneration of these forests. The landscape-scale assessment based on satellite data indicated that high burn severity was driven predominantly by spatial patterns in fuels and weather, with prefire vegetation conditions having the greatest relative influence on high burn severity overall. The mixture of live and dead fuels associated with post-outbreak landscapes and characterized by remote sensing indices suggested that intermediate levels of outbreak generate a fuel arrangement that supports the greatest probability of high severity fire on these landscapes.
My small field study focused on the chemical characterization of carbon legacies generated by MPB and wildfire showed that the concentration of pyrogenic carbon was higher in samples from charred snags compared to scorched snags. These findings indicate a difference in the carbon legacy that remains on landscapes where fire burns through forests with substantial prefire tree mortality versus those where fire burns through stands dominated by live trees.
Overall, this research provides insights into the disturbance and fire ecology for forests dominated by lodgepole pine with substantial prefire tree mortality from previous, recent disturbance. These forests with pervasive tree mortality burn differently than forests dominated by live trees, leaving an altered legacy structure, but dominant species, lodgepole pine, is generally resilient when MPB outbreaks and wildfire occur in short intervals. This research can inform future research and forest management as it relates to fire severity and forest resilience at the epicenter of the MPB outbreaks.